Hypothesis: Leptin is an important regulator of the intestinal inflammatory response to infection, and does so through its effects on either hematopoietic or non-hematopoietic cells of the gut. Leptin and leptin receptor are expressed in the intestinal epithelium and in infiltrating mononuclear cells in the lamina propria. Leptin controls expression of epithelial sodium/glucose and peptide transporters, regulates apoptosis, and induces intestinal inflammation via T lymphocytes. We and others have observed that mice deficient in leptin (ob/ob) or functional leptin receptor (db/db) have altered susceptibility to amebiasis and Clostridium difficile, as well as experimentally-induced inflammatory bowel disease. These results suggest that leptin may have broad regulatory roles in enteric infection and inflammation. Intriguingly, a common (present in one half of the CephUtah population analyzed by HapMap) single amino acid polymorphismm in the leptin receptor (that alters its affinity for leptin) is associated with resistance in children to amebiasis, and in adults to amebic liver abscess. We propose to study the mechanisms by which leptin and its receptor regulate intestinal defense against infection in mice and in humans. First we will test how general the observation is of the link of leptin to intestinal infection and inflammation. With collaborating investigators within MARGE we will determine if ob/ob (leptin deficient) and db/db (leptin receptor deficient) mice have increased susceptibility to infection and inflammation due to Giardia lamblia, Cryptospordium pan/urn, enteroaggregative E. coli and C. difficile. We will then determine the contribution to infection and inflammation of leptin and leptin receptor in intestinal epithelium vs. bone marrow-derived cells for a single infectious agent, and its dependency upon STATS signaling. Finally we will extend these observations to humans by testing for associations of protection from Giardia lamblia and enteroaggregative E. coli with the leptin receptor polymorphism found to be protective from amebiasis.